Vehicle engine fuel systems may use a fuel vapor storage and purging system to reduce evaporative emissions. The system may include an adsorbent-filled canister in communication with a fuel tank, the adsorbent in the canister adsorbing fuel vapors from the fuel tank. Periodically, the system may initiate a canister purge, drawing fresh air into the adsorbent canister. This action causes adsorbed fuel in the canister to desorb and to flow as vapor into the engine intake.
One example approach for controlling fuel vapor purging is described in U.S. Pat. No. 6,237,574. Specifically, an approach is described for improving air-fuel ratio control during fuel vapor purging by smoothing the fuel-vapor spikes that occur on purging a saturated adsorbent canister when the fuel tank is simultaneously full of fuel vapor. The adsorbent canister described therein is configurable such that some of the adsorbent can be used to buffer fuel vapors drawn directly from the fuel tank.
While buffer-based methods may improve control of the air-fuel mixture under purge conditions, they may reduce the ability of the system to purge a sufficient quantity of vapors, thereby leading to increased purging time. Such increased purging time, however, may not be available due to other system requirements, such as manifold vacuum levels, adaptive learning, engine and/or cylinder deactivation, electric-propulsion operation, etc. The inventors herein have recognized the above issues and developed various approaches that may be use in addition to, or in the alternative to, such approaches.